1. Field of the Invention
The present invention relates to a lens system, which is inexpensive, has adequate telecentricity and a zoom ratio of approximately 4.0, to be used in a small and light-weight digital camera, etc.
2. Description of the Prior Art
In recent years, further miniaturization of digital compact cameras due to further miniaturization of electronic components have progressed, and further miniaturization of the photographing optical system is also in demand.
Furthermore, due to higher pixelization of imaging devices, photographing optical systems are required to have higher resolution.
In order to prevent shading and color shift, excellent telecentricity, by which light emitting from the rearmost lens surface is orthogonally incident on the imaging surface, is required in the photographing optical system of a digital camera.
A negative-lead type lens system having a zoom ratio of approximately 3 is often utilized as the zoom lens system of a compact digital camera. Since a negative-lead type lens system can achieve a wide angle-of-view at the short focal length extremity, and can have smaller lens diameters, especially, the diameter of the most object-side lens group, such a lens system is suitable for a retractable zoom lens system in which the distances between lens groups are reduced as the lens groups are being retracted (in an optical axis direction) to an accommodating position.
Furthermore, such a zoom lens system having three lens groups, e.g., a lens group having a negative refractive power (hereinafter, a negative lens group), a lens group having a positive refractive power (hereinafter, a positive lens group) and a positive lens group, in this order from the object, is often utilized, since the exit pupil has to be positioned sufficiently away from the imaging plane.
Examples of the prior art can be found in Japanese Unexamined Patent Publication Nos. 2005-70696, 2005-70697, 2005-37727 and 2006-301154.
In the case of a zoom lens system having a zoom ratio exceeding 4.0, it is common to employ a positive-lead type lens system. Such a positive-lead type lens system is suitable for reducing the overall length of the lens system; however, the diameter of the first lens group is larger, and is not suitable for the retractable zoom lens camera with a telescopic lens barrel which is arranged to advance and retract.
Japanese Unexamined Patent Publication Nos. 2005-70696, 2005-70697 and 2005-37727 disclose relatively small optical systems; however, the zoom ratio thereof is approximately 3.0, which is insufficient. Furthermore, cost reduction of the zoom lens systems has not been sufficiently achieved.
In Japanese Unexamined Patent Publication No. 2006-301154, a zoom lens system in which a zoom ratio exceeding 4 is proposed; however, the overall length of the zoom lens system is long, so that miniaturization thereof is insufficient. Moreover, a large number of lens elements, and glass aspherical lens elements are employed, which increases the costs of the optical system.
Generally, if attempts are made to achieve an optical system in which aberrations are suitably corrected without increasing the size of the optical system, the number of lens elements has to be increased, and more aspherical lens elements have to be employed, which increases the costs of the optical system.
In a zoom lens system for a telescopic retractable zoom lens camera, a negative-lead type lens system is desirable. However, if the zoom ratio is increased up to approximately 4.0, the zoom lens system tends to be longer in the optical axis direction, and the correcting of aberrations becomes difficult.